1. Field of the Invention
The present invention relates to a Mach-Zehnder type semiconductor device and a method of controlling the same.
2. Description of the Related Art
A Mach-Zehnder type optical modulator usable for an optical communication system and an optical information processing system is disclosed in, for example, “Japanese Patent Kokai No. 2000-28979” where a LiNbO3 crystal is used as a material for the modulator.
A Mach-Zehnder type optical modulator using semiconductor materials is advantageous in miniaturization because the device length using semiconductor materials is one several tenth of that using the LiNbO3 crystal, and also is advantageous because of its easiness for integration with other optical devices.
A conventional MZ type semiconductor device disclosed in, for example, “Claude Rolland et al., InGaAsP-based Mach-Zehnder modulators for high-speed transmission systems”, Optical Fiber Communication Conference 1998 (OFC'98), p. 283-284. (a document D1) will now be described with reference to the accompanying drawings.
FIGS. 1A and 1B represent schematic drawings which show a conventional MZ type semiconductor device. FIG. 1A is a plan view showing the conventional MZ type semiconductor device. FIG. 1B is a cross-section view showing a cross-section of the MZ type semiconductor device taken along A-A line in FIG. 1A.
The conventional MZ type semiconductor device 100 disclosed in the document D1 has a MZ (Mach-Zehnder) type modulator 110 and a DFB (Distributed Feedback) laser 120, both of which are formed on a InP substrate 22 having n-type conductivity (an n-InP substrate) as a semiconductor substrate. A laser light emitted by the DFB laser 120 is incident to the MZ type modulator 110.
The MZ type modulator 110 has an n-InP substrate 22, a waveguide layer 30 formed on the n-InP substrate 22, and a p-InP layer 26 formed on the waveguide layer 30. It is to be noted that the p-InP layer 26 is not shown in FIG. 1A and a cap layer etc. for protecting the modulator is not also shown in FIGS. 1A and 1B for the sake of simplicity.
The waveguide layer 30 includes an entrance waveguide 32, an optical splitter 33, a first branch waveguide 34, a second branch waveguide 36, an optical coupler 37, and an exit waveguide 38. The entrance waveguide 32 guides propagation of the laser light emitted by the DFB laser 120 as an incident light to the optical splitter 33. The optical splitter 33 connected to the entrance waveguide 32 splits the laser light transmitting through the entrance waveguide 32 into first and second split lights. The first branch waveguide 34, which is connected to the optical splitter 33 and the optical coupler 37, guides propagation of the first split light passing through the optical splitter 33 to the optical coupler 37. The second branch waveguide 36, which is connected to the optical splitter 33 and the optical coupler 37, guides propagation of the second split light passing through the optical splitter 33 to the optical coupler 37. The optical coupler 37 recombines the first and second split lights passing through the first branch waveguide 34 and the second branch waveguide 36, respectively, so as to generate an outgoing light. The exit waveguide 38 connected to the optical coupler 37 guides propagation of the outgoing light passing through optical coupler 37. The outgoing light passing through the exit waveguide 38 is emitted by the MZ type modulator 110.
The MZ type modulator 110, in which the first and second branch waveguides 34 and 36 have the same path length, will be described. It should be noted that the path length is defined as a geometrical length of the waveguide. The first and second split lights, which are split by the optical splitter 33, transmit through the first and second branch waveguides 34 and 36, respectively, and the two split lights are recombined by the optical coupler 37. A phase difference between the first and second split lights recombined by the optical coupler 37 is 0 degrees.
A first modulating electrode 42 is provided above the first branch waveguide 34. A refraction index of the first branch waveguide 34 which exists under the first modulating electrode 42 is changed in response to a voltage applied to the first modulating electrode 42. A second modulating electrode. 44 is provided above the second branch waveguide 36 similarly to the first modulating electrode 42. A refraction index of the second branch waveguide 36 under the second modulating electrode 44 is changed in response to a voltage applied to the second modulating electrode 44.
A voltage is applied to either one of the first modulating electrode 42 or the second modulating electrode 44 or two different voltages are applied to the first modulating electrode 42 and the second modulating electrode 44, respectively, thus causing an electric potential difference between the first modulating electrode 42 and the second modulating electrode 44. Thereby, the reflection indexes of the first and second branch waveguides 34 and 36 are changed, the values of which are different from each other. Each optical length of the first branch waveguide 34 and the second branch waveguide 36, through which the first split light and the second split light travel, respectively, is given by a product of the path length and the refraction index, as is well-known. If the refraction indexes of the first branch waveguide and the second branch waveguide are different from each other, a phase difference between the first split light and the second split light causes when the optical coupler 37 recombines the two lights. As a result, the optical intensity of the outgoing light, which is recombined with the first and second split lights by the optical coupler 37, changes in accordance with the phase difference.
In the case that the phase difference between the recombined first and second split lights is 180 degrees, the optical intensity of the outgoing light has a minimum, that is, the MZ type modulator 110 is turned off. In the case that the phase difference between the recombined first and second split lights is 0 degrees, the optical intensity of the outgoing light has a maximum, that is, the MZ type modulator 110 is turned on.
However, in the case that the first and second branch waveguides have the lengths different from each other, the modulation voltages required for turning on or off the MZ type modulator fluctuate corresponding to the path lengths of the first and second branch waveguides. For instance, if a light having a wavelength of 1.55 micro meters and the waveguide having a reflection index of 3.2, which is a typical value of a waveguide formed from semiconductor materials, are given, a path length of the waveguide corresponding to the phase difference of 180 degrees is 0.242 (=1.55/3.2/2) micro meters. In order to suppress fluctuations in the modulation voltages of the device, it is necessary to that a difference ΔL in the path length between the waveguides is reduced up to about one tenth of the path length of the waveguide corresponding to the phase difference of 180 degrees, that is, 0.0242 micro meters. A typical path length from the optical splitter to the optical coupler is longer than 600 micro meters. Thus, it is extremely difficult to form waveguides in which the difference in the path length between the waveguides is 0.0242 micro meters because the path lengths are designed to an accuracy of 0.04% and more of the difference in the path length between the waveguides.
The fluctuations in the phases of the two lights respectively transmitting through the two waveguides between which there is the difference in the path length are conventionally adjusted by applying a modulation voltage in addition to a direct current (DC) bias voltage to modulating electrodes.
The conventional phase adjustment effect will be described with reference to FIGS. 2A, 2B, and 2C. FIGS. 2A, 2B, and 2C are graphs showing optical properties to describe the phase adjustment effect, each representing a relation between optical loss in intensity of the outgoing light and modulation voltages applied to the first modulating electrode and the second modulating electrode. Each horizontal axis of FIGS. 2A, 2B, and 2C represents a first voltage V1 (volts) applied to the first modulating electrode as a modulation voltage, while each vertical axis represents optical loss (dB) in intensity of the outgoing light for the conventional MZ type modulator. Curves I, II, III, and IV show optical loss profiles of the outgoing light emitted when the second voltages applied to the second modulating electrode are 0, −0.5, −1, and −1.5 volts, respectively. It should be noted that the path lengths of the first and second branch waveguides are so designed that the MZ type modulator is turned on when the first and second voltages V1 and V2 are 0 volts and turned off when the first and second voltages V1 and V2 are −4 and 0 volts, respectively.
FIG. 2A shows a relation between optical loss of the outgoing light and modulating voltages applied to the first and second modulating electrodes for the MZ modulator in which path lengths of the first branch waveguide. and the second branch waveguide are formed as they are designed. The curvature I (V2=0 volts) has a minimum (−30 dB) at the first voltage V1 of 0 volts, indicating that the MZ type optical modulator is turned off. On the other hand, the curvature I has a maximum (−10 dB) around at the first voltage of −4 volts, indicating that the MZ type optical modulator is turned on. As increasing the second voltage V2, the first voltage V1 at which the MZ type modulator is turned off increases as denoted by an arrow V in FIG. 2A. As described, the first voltage V1 at which the MZ type optical modulator is turned off can be adjusted by changing the second voltage V2.
However, there is a problem as follows. As increasing the first and second voltages V1 and V2 applied to the first and second modulation terminals, respectively, the first and second split lights are likely to loss in intensity due to an electro-optical effect when transmitting through regions of the first and second waveguides where the first and second voltages are applied. FIG. 3 is an optical characteristic for representing intensity loss of the outgoing light as a function of the first voltage V1. The vertical axis represents optical loss in intensity of the outgoing light (dB) and the horizontal axis represents the first voltage V1 (volts). FIG. 3 is a calculation result of the outgoing light as a function of the first voltage V1 for the MZ type modulator whose device length is 600 micro meters. As increasing the negative first voltage V1, the loss in intensity gradually increases and drastically increases around at the first voltage of −4 volts. As increasing the phase difference, the optical loss in intensity of the light increases when the phase difference is adjusted. Thus, the optical loss in intensity of the light increases when the device is turned on. It is indicated from V1 denoted in FIG. 2A that as increasing the absolute value of the second voltage V2, the loss of the light increases in intensity at the first voltage V1 of −4 volts at which the modulator is turned on.
FIG. 2B shows a relation between optical loss in intensity of the outgoing light and modulating voltages applied to the first and second modulating electrodes for the MZ type modulator in which path lengths of the first branch waveguide and the second branch waveguide are deviated from those designed values. The MZ type modulator is turned off at the second voltage V2 of 0 volts (curve I) and the first voltage V1 in a range of a positive voltage, that is, when the optical intensity loss has a minimum point (not shown in FIG. 2B). As decreasing the second voltage V2, the first voltage V1 at which the MZ type modulator is turned off decreases. The MZ type modulator is turned off at the second voltage V2 of −1.5 volts (curve IV) and the first voltage V1 of about 0 volts. On the other hand, the MZ type modulator is turned on at the second voltage V2 of −1.5 volts (curve IV) and the first voltage V1 of about 4 volts. The MZ type modulator, to which the first and second voltages shown in FIG. 2B are applied, can be operated by setting the second voltage V2 to a favorable value.
In the case that the second voltage of −1.5 volts is applied, optical intensity loss of light transmitting through at a region of the waveguide where the second voltage is applied is increased due to the electro-optical effect. Therefore, the loss in intensity of the outgoing light produced when the MZ modulator is turned on is larger than that shown in FIG. 2A.
FIG. 2C shows a relation between optical loss in intensity of the outgoing light and the modulation voltage applied to the first and second modulation electrodes for the MZ type modulator in which a pass difference between the two waveguides is longer than that of the case shown in FIG. 2B. It is required that the second voltage V2 is in a range of −3 to −4 volts to modify a phase difference (not shown in FIG. 2C). The loss in intensity of the outgoing light is so large that the MZ type modulator is not for practical use.
As described above, the MZ type modulator where the phase difference between the first and second split lights is large beyond necessity is impractical.
Inventors applying for this application of the present invention devote themselves to an investigation for a MZ modulator that can solve the above-mentioned problems and found that the phase of the two lights can be adjusted by applying a voltage added to a positive bias voltage as a modulation voltage.
A phase adjusting effect will be now described with reference to FIG. 4. FIG. 4 is a graph showing a first voltage V1 versus a second voltage V2 for the MZ type semiconductor device 100 shown in FIG. 1A. The first and second voltages V1 and V2 are applied to the first and second modulating electrodes 42 and 44, respectively. Vertical and horizontal axes of the graph represent the first voltage V1 (volts) at which the MZ type semiconductor device 100 is turned off and the second voltage V2 (volts).
The first voltage V1 at which the MZ type modulator 100 is turned off is −4.7 volts when the second voltage V2 of −3 volts is applied. As increasing the second voltage V2, that is, as the second voltage V2 approaching to 0 volts, the first voltage V1 at which the MZ type modulator 100 is turned off gradually approaches to 0 volts. As further increasing the second voltage V2, the first voltage V1 at which the MZ type modulator 100 is turned off suddenly increases around at the second voltage V2 of 0.5 volts. This phenomenon originates in a plasma effect caused by the carriers injected into the waveguide layer.
In general, a change of reflection index originating in the plasma effect is larger than that originating in the electro-optical effect. Therefore, if the positive bias voltage is applied, the first voltage V1 at which the MZ type modulator becomes an off state changes more largely, indicating that the MZ type modulator has a high phase adjustment function. Furthermore, a light losses in intensity due to the electro-optical effect in the case that a negative bias voltage is applied, whereas a light does not loss in intensity due to the electro-optical effect in the case that the positive bias voltage is applied. As a result, the phase adjustment can be efficiently performed.
However, there is a possibility that the carriers injected into the waveguide by applying the positive bias voltage leak from a region where the positive bias voltage was applied, thus causing deterioration in an eye pattern of the outgoing light.
FIGS. 5A and 5B are drawings for describing a carrier injection. FIG. 5A shows the carrier injection schematically. A horizontal axis of FIG. 5B represents a position in the direction of a light and a horizontal axis of FIG. 5B represents an electric potential V. FIGS. 5A and 5B shows the carrier injection for the conventional MZ type semiconductor device described with reference to FIG. 1A similar to FIG. 4 in the case that the positive bias voltage is applied to the first modulating electrode 42. By applying the positive bias voltage to the first modulating electrode 42, carriers are injected into a region of the waveguide layer 30 which exists under the first modulating electrode 42. The carriers diffuse out a region of the first branch waveguide 34 which exists under the first modulating electrode 42. If the carriers are modulated under the second modulation voltage applied to the second modulating electrode 44, an eye pattern of the outgoing light will be disturbed.